Polythiol accelerated radiation cross-linking of olefinically unsaturated polymers

ABSTRACT

Improved radiation vulcanization of elastomers is accomplished by incorporating into the elastomer composition a polythiol. The polythiol is preferably a hydrocarbon thioether thiol and is normally liquid at room temperature with a molecular weight of at least 150. Though difunctional polythiols are operative in this invention, the preferred polythiols have about three to about five thiol groups per molecule.

Unite States Patent [191 Zapp et a1.

1 Feb. 4, 1975 POLYTI-1IOL ACCELERATED RADIATION CROSS-LINKING OF OLEFINICALLY UNSATURATED POLYMERS [75] Inventors: Robert L. Zapp, Short Hills; Alexis A. Oswald, Mountainside, both of NJ.

[73] Assignee: Exxon Research and Engineering Company, Linden, NJ.

22 Filed: Mar. 6, 1972 211 App1.No.:232,275

[52] US. Cl... 204/159.18, 204/l59.l4, 204/159.15, 260/23 XA, 260/23 H, 260/235, 260/79, 260/795 L, 260/795 P, 260/795 NV UNITED STATES PATENTS 3,338,810 8/1967 Warner 204/159.18

3,625,925 12/1971 Oswald et a1. 204/l59.18 3,662,023 5/1972 Kehr et a1. 260/858 3,700,574 10/1972 Kehr et al 204/159.l5 3,725,229 4/1973 Kehr et a1. 204/159.l5

Primary Examiner-Paul Lieberman Assistant Examiner-Richard B. Turner [57] ABSTRACT Improved radiation vulcanization of elastomers is accomplished by incorporating into the elastomer composition a polythiol. The polythiol is preferably a hydrocarbon thioether thiol and is normally liquid at room temperature with a molecular weight of at least 150. Though difunctional polythiols are operative in this invention, the preferred polythiols have about three to about five thiol groups per molecule.

15 Claims, N0 Drawings POLYTHIOL ACCELERATED RADIATION CROSS-LINKING OF OLEFINICALLY UNSATURATED POLYMERS BACKGROUND OF THE INVENTION Radiation curing of polymers is well known in the art. Though difficulties have been encountered in radiation curing or vulcanizing of polymers, improved results have been obtained by the use of additives. For example, in the case of polyethylene, it has been demonstrated that the addition of unsaturated polyfunctional monomers will produce comparable cross-link densities at reduced dose levels, in contrast to the radiation of ordinary polyethylene.

It is well known that polymers such as polyisobutylene are effectively destroyed when exposed to ionizing radiation. See for example, Calorimetric Study of Radiation Crosslinking of Polyisobutylene In The Presence Of Monomer Additives, Dokl. Akad. Nauk SSSR (Phys. Chem.) 193 (4), 855-857 (1970). The addition of small amounts of polyfunctional monomers such as p-divinylbenzene causes effective crosslinking of polyisobutylene as does the addition of acrylonitrile to polyisobutylene blends. Allyl acrylate has also been found to cause crosslinking of polyisobutylene under ionizing radiation conditions, see for example Polymer Letters, 2, pages 819-821 (1964). Although crosslinking does occur, degradation of the polyisobutyleneresults even in the presence of the allyl acrylates or allyl methacrylates at higher dose levels, e.g., above about 0.8 megarads.

Butyl rubber being substantially polyisobutylene also, not surprisingly, has been found to degrade in the presence of ionizing radiation.

The use of dithiols in the conventional curing of polymers is well known in the art. For example, the heat curing of a cellulose methacrylate derivative with simple dithiols is disclosed in British Pat. No. 588,018. Sulfur vulcanization of styrenebutadiene is accelerated by the addition of bis-mercaptophenyl diphenyl oxide (see, for example, U.S. Pat. No. 3,326,822). Chlorinated butyl rubber has been vulcanized with dithiols. See, for example, Hodges, Rubber Plastics Weekly 141 pages 666-668 (196) wherein glycol dimercapto acetate is recommended because of its low odor.

Elastomers have been prepared from long-chain dithiols with long-chain diolefins using as the crosslinking agent triolefin or long-chain trithiols; see, for example, Klotz et al, I and EC Product Research and Development 7, pages 165-169 September (1968).

SUMMARY OF THE INVENTION It has been found that olefinically unsaturated elastomers, particularly halogenated butyl rubber, styrenebutadiene rubber, polybutadiene, etc., may be vulcanized using ionizing radiation. Surprisingly, the conveying radiation induced vulcanization of halogenated butyl rubber and polydiene rubbers can be markedly accelerated by the addition of polythiols into the clastomer composition prior to exposure to radiation. The polythiols which are suitable for use in this invention are polyfunctional thiols having at least 2 preferably 3 thiol groups. The polythiol is preferably a liquid low molecular weight compound having a molecular weight of at least 150.

The preferred polythiols are polythiol ethers or other hydrocarbon polythiols. Other types of functionality may be introduced into the molecule such as carbonyl groups, etc.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method of radiation vulcanization of olefmically unsaturated elastomers such as halogenated butyl rubber polybutadiene and styrene butadiene rubber (SBR), which comprise incorporating into the elastomer composition a crosslinking agent wherein the crosslinking agent is a polythiol. In particular this invention relates to the improvement in radiation vulcanization of polybutadiene, halogenated butyl rubber and SBR wherein the polythiol has an average of at least three thiol groups and is normally liquid at room temperature having a molecular weight of at least 150 and preferably less than 3,000. More preferably, the polythiol is normally liquid at ambient temperatures.

The term butyl rubber as employed in the specification and claims is.intended to include copolymers made from the polymerization of reactant mixtures having therein about to about 99.5 percent by weight of an isoolefin which has about 4 to 7 carbon atoms, e.g., isobutylene, and about 0.5 to 30 percent by weight of a conjugated multiolefin having about 4 to about 14 carbon atoms, e.g., isoprene, piperylene, cyclopentadiene, etc. The resulting copolymer contains to 99.8 percent by weight of combined isoolefin, and about 0.5 to about l5 percent of combined multiolefin. Butyl rubber generally has a Staudinger molecular weight of about'20,000 to about 500,000, preferably about 25,000 to about 400,000, especially about 100,000 to about 400,000, Wijs Iodine number of about 0.5 to about 50, preferably about 1 to about 15.

The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128 which is incorporated herein by reference.

For the purpose of this invention, the butyl rubber may have incorporated therein about 0.2 to about 10 mole percent of combined multiolefin, preferably about 0.5 to about 6 percent, more preferably about 1 to about 4 percent, e.g., 2 percent. Illustrative of such a butyl rubber is Enjay Butyl 268 (Enjay Chemical Company) having viscosity average molecular weight of about 450,000, a mole percent unsaturation of about l.5 percent and a Mooney viscosity of about 55 at 260F.

Halogenated butyl rubber is commercially available and may be prepared by halogenating butyl rubber in a solution containing about 1 to about 60 percent by weight of butyl rubber in a substantially inert C -C hydrocarbon solvent, such as pentane, hexane, heptane, etc., and contacting this butyl rubber cement with a halogen gas for a period of about 25 minutes, whereby halogenated butyl rubber and a hydrogen halide are formed; the copolymer containing up to l halogen atom per double bond in the copolymer.

The preparation of halogenated butyl rubber is old in the art, see for example U.S. Pat. No. 3,099,644 which is incorporated herein by reference. This invention is not intended to be limited in any way by the manner in which butyl rubber is halogenated, and both chlorinated and brominated butyl rubber are suitable for use in this invention.

Illustrative of halogenated butyl rubbers is Enjay Butyl HT-l0-66, a chlorinated butyl rubber containing about 1.3 weight percent chlorine having about 1.7

3 4 mole percent unsaturation and a viscosity average mo-v Any polythiol having at least two thiol groups which lecular weight of about 357,000. has a molecular weight of at least 150, may be used in Styrene butadiene rubber (SBR) is well known to the the practice of this invention. Although the polythiol art. Typically these polymers contain about 5 to about structure may include other atoms besides carbon, hy- 40 weight percent of styrene and have a Wijs Iodine 5 drogen, and sulfur; e.g., nitrogen, or oxygen in the form N b of about 300 to about 5,000 d a number vof amides, ethers, esters, etc., the preferred polythiols erage molecular i h f b t 30,000 to ab t are hydrocarbon polythiol others. The preferred poly- 600,0()(), f bl about 100,000 to b t 150,000, thiols have a molecular weight of about 150 to about The polymer may be prepared either by the bulk poly- 3,000 preferably and have at least 2, preferably about merization of styrene and butadiene or polymerization 3 to about 5 thlol g p P molecule; more preferaof the rubber in a letex form by emulsion polymerizay about 200 to about tion, a suspension polymerization. This invention is in Illustrative examples of these thiols are the reaction no way intended to be limited by the method of prepaproduct of trivinylcyclohexane and 1,3-propanedithiol, ration of the styrenebutadiene rubber. which has the general formula s {CHZCHQCHQ son CH2 s{}( CHQCHQSCHQCHQ cn sngg Polybutadiene is well known in the art and may be Trimethylol propane tris-thiolglycoate, pentaerythritol prepared in various ways including butyl lithium catalytetra 3, mercaptopropionate, pentaerythritol sis of butadiene. Illustrative examples of polybutadiene tris-3-mercaptopropionate, the reaction product of cyare: Firestone Diene 55 having 45 percent 1,4 cis enclododecatriene and 1,3-propanedithiol, which is a chainment, 45 percent 1,4 trans enchainment and 10 mixture of thioether thiols having the general formula EBEQSCH CH CH SH) 3 and (Cl-I 8H Cale M01 Wt 187 S(CH -s Cale. M01 Wt. 757

percent 1,2 vinyl pendant groups; Phillips Cis 4 having Trimethylol propane tris-mercaptoacetate, the reaction 92 percent 1,4 cis, 2 percent 1,2 adduct of butadiene product of H 8 and/or dithiols with polyepoxides. See

and some 1,4 trans enchainment. US. Pat. No. 3,625,925 incorporated herein by refer- Other elastomers suitable for use in the practice of once for other suitable polythiols and their method of this invention include polychloroprene and acrylonipreparation. trile-butadiene copolymer. The advantages of the instant invention may be more The term ionizing radiation as used in the specifireadily appreciated by reference to the following examcation and claims is intended to include gamma radiaples. tion and electron beam (,Bray) irradiation.

The C0 gamma radiation source referred to in the EXAMPLE 1 Examples of this specification is a 2,900 Curie source (1.2 Mev). Electron beam irradiation was produced A polythioether polythiol, formed by the reaction from a Dynamitron. For a description of this equipproduct oftrivinylcyclohexane and 1,3-propane dithiol om, see Harmer, and Ballentine Radiation Processwas used as the vulcanizing crosslinking agent. This ing," Chemical Engineering, pages 98-116 (Apr. 19, compound has the general formula s {CH2CH2CH2SCH2CH2S-0 ca cH scn cH cH sm 2 1971). Although gamma radiation has greater pene- The calculated molecular weight of the polythiol is trating power than electron beams, this characteristic 912, but the actual molecular weight determined by was not a factor in these experiments since the test vapor pressure osmometry was 818. This product was samples were 100 mil sheets or less. Where significant designated E2323 lll. It contains 0.0049 mole equivadifferences occur between the results from Co and lents of SH per gram. electron beam irradiation, it is attributable to differ- 60 The polythiol was compounded with SBR and natural enCeS in dose rate, the former at g l'ad/minute rubber (NR) according to the formulations shown in and the latter at 30 megarad/minute. Table 1,

TABLE I POLYTHIOETHER POLYTHIOL lN SBR AND NATURAL RUBBER Component 1 2 3 4 5 6 SBR 1502 100 100 Natural Rubber I00 100 100 Zinc Oxide 5 5 5 5 5 5 Stearic Acid l l l I l l l TABLE I Continued POLYTHIOETHER POLYTHIOL 1N SBR AND NATURAL RUBBER Component 1 2 3 4 5 6 E2323 lll 2.3 4.6 2.3 4.6 Moles SH/Gram 0.011 0.022 0.011 0.022

Elastomer Emulsion pnlymcrimd at 50F Comprising about 23.5% styrene having a Mooney Viscosity at 212! 01' 52 All amounts as parts per hundred of clnstomcrs by weight.

It was observed in other radiation curing experiments that the presence of zinc oxide provided an added accelerating effect to that of the polythiol. Thus, the control compounds in both elastomer blends contain this ingredient. Sample compounds were molded at 175F for 5 minutes to form a 0.025 inch film protected with Mylar; prior to radiation all samples were soluble in cyclohexane. Samples were then exposed to electron beam radiation at 30 megarads per minute for 2.5, 5, l0, and MR. Network properties were determined by volume swelling in cyclohexane and converted to the parameters of Me and XL/cc, wherein Mc is Molecular weight between crosslinks and XL/cc is crosslinks per cc. The results are shown in Table 11.

TABLE ll EXAMPLE 2 Chlorinated butyl rubber under radiation tends to quickly reach a crosslinking limit where further radiation begins to promote chain scission. It is also dose rate sensitive so that as the rate ofenergy absorption is Effect of Electron Beam Exposure Physical Properties at Electron Beam SBR NR Exposure of: l 2 3 4 5 6 0 Megarads Soluble Soluble Vol. Increase 443 328 Loose Gel Soluble Loose 10.4 9.2 Mc X 10*" Gel 9.2 6.0 XL/cc X 10" 2.69 4.42

(Orig. Pol.

Vol.)

7: Vol. Increase 2300 316 242 Loose 2765 2420 7: Soluble 7.7 7.0 Gel 26.3 24.6 Mc X 10* 110 5.6 3.9 135 102 XL/cc X 10 .14 4.63 6.7 .16 .20 10 MR Vol. Increase 949 279 202 1940 1078 1006 Soluble 19.6 7.0 6.3 20.1 11.7 11.4 Mc X 10 3 29 4.7 2.9 .78 32 29 XL/cc X 10" 0.76 5.48 8.9 .30 .81 .89 15 MR Vol. Increase 595 251 187 1170 752 716 Soluble 12.8 6.5 5.9 12.1 7.9 8.0 Mc X 10 14.2 4.0 2.7 36 18 16.5 XL/cc X 10 1.68 6.53 9.71 .71 1.47 1.57

increased, reaction selectivity is shifted from crosslinking to chain scission. In contrast to more highly unsaturated general purpose elastomers, a mild radiation precure ofa compounded formulation detracts from a subsequent thermal curing process. The radiation-induced crosslinking of chlorinated butyl rubber can be greatly enhanced by small amounts of polythioether polythiols as illustrated by the following experiments.

The polythiol used is the reaction product of cyclododecatriene and 1,3 propane dithiol, designated E1255 VII. The polythiol is a mixture of the following much less pronounced but still evident on the basis of compounds:

calc. mol. wt. 487

calc. mol. wt.

TABLE III Chlorinated butyl rubber HT 10-68 100 100 100 100 Zinc Oxide 5 5 5 Glycol Dimercapto Acetate 2 E1255 VII 2 chlorinated butyl rubber of about 450,000 viscosity average molecular weight having 1.3 weight I! chlorine and 1.7 mole l: unsaturation.

The chlorinated butyl rubber used has approximately 0.032 equivalent Cl functionality per 100' grams. Two grams of El255 Vll polythiol is the equivalent to the addition of 0.01 SH groups per 100 grams of chlorinated butyl rubber. One hundred grams of chlorinated butyl rubber contain 0.032 allylic chlorine groups.

posed to Cobalt 60 radiation for various time periods. After exposure, the film was subjected to solvent swelling'analysis in a cyclohexane to determine the extent of crosslinking. The results are tabulated in Table IV.

. -A comparison of columns l and 2 shows that zinc oxide has a very mild activating effect upon the radiation-induced crosslinking of chlorinated butyl rubber. The presence of glycol dimercapto acetate provides little or no additional benefit (column 3). On the other hand, the addition of El255 Vll polythiol (column 4) provides a marked enhancement or radiation-induced erosslinking. By any criterion: lower volume swelling, soluble polymer, molecular weight Me or crosslinked density (XL/cc), the polythioether polythiol has a marked accelerating effect. On the basis of crosslinks formed, it provides about a threefold increase over the control of column 2 and an increase of about 2-V2 times over the dimercaptan of column 3.

EXAMPLE 3 In order to study the concentration effect of El255 VII polythiol of Example 2 on radiation curing, samples were prepared in the manner of Example 2 at various polythiol concentrations. The samples were molded at 175F. for 5 minutes. The formulations are shown in Table V.

Note: All samples were soluble in cyclohexane before irradiation.

TABLE IV SWELLING IN CYCLOHEXANE AT ROOM TEMPERATURE AS A FUNCTION OF MEGARAD DOSE (MR), CO IRRADIATION Physical Properties after Co I 2 3 4 0 MR (Megarads) Soluble Soluble Soluble Soluble 4.3 MR Vol. Increase 607 Soluble 3.7

Mc X 10' (Gel) 14.2

XL/cc X 10" (Orig. 1.87

Volume Basis) 5.5 MR Vol. Increase 1322 1339 1162 516 Sol. 10.4 8.8 9.6 3.7

Mc X 10" 54 41 10.8

XL/cc X 10" 0.51 0.50 0.64 2.45

10 5 MR Vol. Increase 970 870 854 470 Me X 10* 30 25.5 24.5 8.6

XL/ec X 10"" 0.78 1.02 1.07 2.83

15.2 MR Vol. Increase 810 740 751 444 Mc X 10'" 22.5 19.5 20 8.6

XL/cc X 10" 1.11 1.32 1.2) 3.1

The compounds of Table III were molded as 0.025 inch at 200F. for 5 minutes between mylar film and ex- The results of Cobalt exposure are tabulated in Table VI. 5

TABLE V1 C Radiation at .01 MR/min.

Network Properties from Volume Swelling Cyclohexane Dose 0 1 2 3' 4 4.7 MR v61. Increase 1138 1203 1040 843 511 Mc x (Gel) 39 43 34 24.5 10.5

XL/cc x 10" .64 .58 .72 1.09 2.45

(Orig. P01. v01.)

10.5 MR v01. Increase 915 763 720 624 447 Sol. 10.3 8.0 8.0 6.8 4.4

Mc x 10- 27 20 18.5 15 8.2

XL/cc 10- .94 1.26 1.38 1.73 3.08 15.3 MR v01. Increase 727 645 654 566 406 Mc x 10-- 19 15.6 15.8 12.5 7.2

XL/cc x 10- 1.31 1.6] 1.59 2.06 3.51

There is a progressive crosslinking response to the 20 EXAMPLE 4 chlorinated butyl rubber system toward radiation as the amount of polythiol is increased. Volume swelling,v sol- The effects of polythioether polythiol E1255 Vll on uble polymer and Mc between crosslinks all decrease the electron beam irradiation response of chlorinated at any given megarad dose level. The generated crossbutyl rubber was evaluated by repeating the experilmked de ity is i s d about 3 t 4 ti v th ments of Example 3, substituting electron beam irradiacontrol column 1 at the highest concentration of ol tion for Cobalt 60 radiation. The results are shown in thiol. Table /ll.

TABLE Vll EFFECT OF POLYTHIOETHER POLYTHlOL (E1255 Vll) lN INCREASING QUANTITIES OF THE ELECTRON BEAM lRRADlATlON RESPONSE OF CHLORINATED BUTYL RUBBER at MR/Min.

- Network Properties from Volume Swelling Cyclohexane Physical Properties after electron beam irradiation at: 0 l 2 3 4 5 5.2 MR Vol.

Increase 1830 1745 1496 1280 737 520 Soluble 15.5 14.0 12.3 11.7 6.9 6.6 Ms X 10 (Gel) 83 76 61 46 19.5 11.0 XL/Cc X 10' 0.29 0.33 .41 .55 1.35 2934 (Orig. Pol.

Vol.) 9.8 MR Vol.

Increase 1270 1182 1085 97 615 435 Soluble 18.9 15.7 16.2 13.9 9.2 7.2 Mc X 10' 47 42 36 31 14.4 8.1 XL/cc X 10 .50 .59 .67 .79 1.78 3.07 15 MR Vol.

Increase 1121 970 923 792 562 423 Soluble 17.2 14.3 14.2 13.3 8.7 7.8 MC X 10' 39 30 28 21.5 12.2 8.3 XL/cc X 10"" 0.62 0.79 0.86 1.13 2.06 3.17

Note: All samples were soluble in cyclohexane before irradiation.

11 12 It is evident that the presence of the polythioether. uated. This rubber had a specification molecular polythiol promotes gelation and retards chain scission. weight ofl .000 and 21 wt. percent mercaptan range of 5.9 to about 7.7 (average 6.8). This is equivalent to EXAMPLE 0.002 SH groups per gram compared to 0.005 SH 5 groups per gram of El255 VII. Samples were prepared A comparison of various polythiols was made using in the manner of Example 4. the polythiols being incorthe polythiols shown in Table VIII. porated in the compositions at equivalent SH function- TABLE VIII POLYTHIOETHER POLYTHIOLS DERIVED FROM TRIENE- DITHIOL POLY iADDITIONS DESCRIBED IN U.S. PAT. APPL. SER. NO. 665,728

Polythiol Reference M01. Wt. Thiol Funct.

No. Calc. Found Cale. Found Chemical Structure and Derivation E1255 v11 487 578 3 2.8 3}( S(CH2)3SH]3 from cycle- (2) do catriene and 1,3 propane dithiol.

E1221 VIII 494 500 4 S[CH CH "81011 011 1 (2) from trivinyl cyclo exane and H S.

2 {3 E1260 v1 796 897 4 [cii scagcn (CI-I CH S(CI-I )gSH) from tr vinyl cyclohexane an ethanol dithiol.

E1262 V1: 529 728 '3 G+cn cn s cn s11] from trivinyl cyclohexane ans iJI- utane dithiol.

' E1263 v11 1698 2 1-IS[(CH SCH 0HS] $CH -5H from methylacetylene and 1,4-butane dithiol.

Polythioether derived from methylacetylene and 1,4-butane dithiol,

the preparation of which is described in U.S. 3,592,798 incorporated herein by reference.

Method for preparing these olythiols are taught in U.S. patent 3,625,925 incorporated herein by reference.

In addition to the polythiols of Table VIII, the Thioality. The compositions and the effect of Cobalt rakol liquid polysulfide dithiol rubber LP3 was also evaldlation on these compositions IS shown in Table IX.

TABLE IX Chlorinated butyl rubber I 00 I00 I 00 I00 I 00 I00 I00 I 00 I 00 Zinc Oxide 5 5 5 5 5 5 5 5 5 Stearic Acid I I I l I l I I I E1255 VII 2 4 E122] VIII 1.3 2.6 E1260 VI 2.3 E1262 VII 2.5 EI263 VIII 9.4 Thiokol LP3 5.0

0 Megarad l00% Sol. Sol. Sol. Sol. Sol. Sol. Sol. Sol. Sol. Co Irradiation 0.01 MR/min.

XL/cc X 10'" 4.2 MR 0.5 2.30 2.02 L68 L 0.75 L0 7.5 MR 2.53 L30 9.4 MR l.l5 2.92 2.59 2.27 L38 I2.4 MR L40 3.25 3.26 3.0 2.51 l.67 1.82 Irradiation by the Electron Beam 30 MR/min.

XL/cc X 10'" 5 MR 0.3I I.35 2.34 L22 2 I4 .72 0 52 0.43 I0 MR 0.60 L 3.07 L61 3 05 I27 I.l8 0 84 0.79 l5 MR 0.77 2.0 3.l7 L92 3 94 .46 I I2 0.88

active hydrogen species would enhance the radiation crosslinking response of chlorinated butyl rubber. lncluded in these studies were mono-, diand trithiols as shown in Table XI.

TABLE Xl Components l 2 3 4 5 6 Chlorinated Butylrubber HT-l-68 Zinc Oxide Stearic Acid l-butane thiol (.Oll

SH/gm). l,4-butane dithiol (.016 SH/gm) .7 1,2.3-propane trithiol (.021 SH/gm) .5 Phenyl Phosphine (.009] F/gm) lOO 100 100 '-l00 100 100 5 5 5 5 5 l l l l l l Diphenyl Silane (.0054

"Parts per I00 parts of elastomcr by weight.

The tetra functional E122] Vlll as shown in columns 4 and 5 is especially noteworthy because of its ability to promote the crosslinking response at quite low concentration levels. The commercial Thiokol rubber has only a mildly activating effect under Cobalt 60 radiation which appears to be further reduced under the higher dose rates of the electron beam.

EXAMPLE 6 The polythioether polythiols of this invention will promote the radiation crosslinking of chlorinated butyl rubber in the absence of zinc oxide and stearic acid as demonstrated by the results in Table X.

Functionalities per gram are shown adjacent to each additive and the amounts added would be comparable to the functionality of 2 parts of El255 Vll added to 100 parts of chlorinated butyl rubber. These obnoxious and sometimes toxic chemicals were added to chlorinated butyl rubber, zinc oxide, stearic acid masterbatch by solution mixing. Solutions were made in n-pentane and solid films recovered by room temperature evaporation in a hood followed by vacuum drying at room temperature. All films prior to irradiation with Co source were soluble in cyclohexane. C-rosslinking densities of the networkswere obtained as a function of dose levels and are shown below in Table Xll.

TABLE Xll XL/cc l0 Orig.

Polymer Volume after 0 MR All Soluble 38 MR .33 Loose .98 2.3 .63 .62

Gel 74 MR L36 .6 1.55 2.0 1.39 1.28 12.3 MB 1.55 .81 1.59 2.2 1.32 1.40

TABLE X A comparison ofthe data in Table Xll shows that the monothiol of column 2 has a retarding effect upon Component" 1 3 3 radiation-induced crosslinking over the control column l. The dithiol has a very modest activating effect. as do 8g{," ='8 '00 the phosphine and to a lesser extent, the silane as Zinc Oxide 5 5 shown in columns 3, 5, and 6. The trithiol reflecting 5:22: o l some of the activity of the polythioether polythiols has Crtisslinked density after Co and electron beam exposure a more Pronounced actlvatlng effect, although It does is FSP XL/cc X of not appear to progress as radiant energy absorbed is in- 22%!"3233'331," volume creased. On a functionality basis, it can be considered 4.2 MR 2.54 2.30 .5 inferior to the several types of polythioether polythiols 9.4 MR 2.47 2.92 l.l5 evaluated ll-l MR 2.41 3.25 1.40 Electron Beam 5 MR 1.50 1.35 0.3 in MR L63 L 0.6 EXAMPLE 8 15 MR L63 2.10 0.77

"'Parls per parts of elastomcr by weight.

Although zinc oxide and stearic acid are not essential for radiation cure, they enhance the effect of the polythiols on radiation cure.

EXAMPLE 7 A series ofexperiments were conducted to determine whether other smaller molecular structures containing Prior experiences have shown that radiation precure of chlorinated butyl rubber subsequently compounded with ingredients that promote chemical thermal curing, generally detracted from the subsequent thermal cure.

Table XIV shows the cffectofpolythiol E1255 Vll on thermal cure after a precure by electron beam exposure.

TABLE XIV Composition: Chlorinated Butyl HT 10-68 100 Zinc Oxide Stearic Acid 1 PTEPT (E1255 Vll) as indicated Crosslinks Generated on Basis of Original Polymer Volume XL/cc X Parts of Polythiol per 100 After Parts of After Electron Thermal Chlorinated Beam Exposure (RC) Cure After TC+RC X 100 Butyl Rubber 5MR 10 MR MR (TC) Only TC+RC TC 0 pt. E1255 29 1.57 1.15 72% Control 5O 1.57 1.06 68 .62 1.57 1.12 7| 1 pt. E1255 .55 2.12 1.97 93% .79 2.12 1.65 78 1 13 2.12 1.59 75 2 pt. E1255 1.35 2.65 2.42 91% 1 78 2.65 2.37 90 2.06 2.65 2.30 87 4 pt. E1255 2.34 3.39 3.90 115% These results show that at 1 pt. per 100 of polythiol there is a subtractive effect on the thermal cure; however, at higher levels the effect is additive.

EXAMPLE 9 Various polythiols were compared for their effectiveness with E1221 VIII polythiol. The three thiols tested in these experiments are as follows:

981 Trimethylol propane tris-thioglycolate Formula weight C H,,C(CH,OCOCH,SH).-, Formula weight 398 Found VPO, Molar equiv. g9 SH/gm 3/398 Molar equiv S gm =0.0076 /5 98-2 Pentaerythritol tetra 3 mcrcaptoproprionate 0 O .0080

C(CH OCOCH CH-,SH).. Formula weight 488 Molar equiv.

SH/gm =4/488 Formulations were prepared and exposed to electron =0.0082 beam irradiation. The results are shown in Table XV.

TABLE XV POLYTHIOL COMPOUNDS AS RADIATION CURE PROM OTERS OF CHLORINATED BUTYL RUBBER EXPOSURE TO ELECTRON BEAM AT 30 MEGARADS PER MINUTE Parts by Weight Component 1 2 3 4 5 6 7 Chlorinated Butyl I00 I00 100 I00 I00 I00 I00 Rubber HT 10-66 Zinc Oxide 5 5 5 5 5 5 5 Stearic Acid 1 1 1 l l l l Polythiol 98-1 I .45 2.89

These two thiol containing ester structures were compared with a polythioether polythiol of similar molecular weight and functionality per gram.

98-3 E1221 ill from trivinylcyclohexane and H 5.

Sample films molded between mylar film at F. 1 minute. Exposed to 5, 10, I5 megarads at 30 MR/minute. Networks characterized by volume swell in cyclohexane: at O megarads samples were soluble.

Additive Control 98-1 98-2 98-3 Exposure MR:

Volume lncrease- 1545 1295 1196 1334 1282 670 634 Soluble Polymer 16.3 13.3 12.4 14.0 13.7 5.6 4.2 Mc X 10' 64 48 42 51 47 16.5 XL/cc X 10" 0.37 0.52 0.59 0.48 0.53 1.56 1.78 (O.P.V.) Exposure 10 MR:

Volume Increase 1168 915 834 897 877 597 495 Soluble Polymer 12.9 14.0 12.5 13.3 12.8 6.1 5.7 Mc X 10' 41 27.5 23.4 27 25.5 13.6 10 XL/cc X 10' 0.45 0.90 1.05 0.95 0.98 1.88 .51 Exposure 15 MR:

% Volume lncrease 1040 817 724 738 712 553 446 Soluble Polymer 12.8 14.3 12.1 12.5 12.2 6.8 4.8 Mc X 10" 39 23 18.2 19.5 18.2 12 8.4 XL X 10" 0.73 1.07 1.32 1.26 1.36 2.06 3.05

With column 1 as the control, the greater effectiveness TABLE XVI of polythiol 98-3 can be observed in columns 6 and 7, as compared to 2 through 5. At the 5 MR exposure level, the ester structures (98-1', 98-2) on the basis of generated crosslinks (XL/cc) provide a response only marginally better than the control, while 98-3 provides an increase in crosslinking response of 5 to 6 times. This advantage is maintained at the higher dose levels. Under Co gamma radiation, at 0.01 MR/minute, the ester structures 98-1, 98-2 displayed a three-fold advantage in crosslinking response when compared to the control with 98-3 providing a further advantage.

EXAMPLE 10 Conventional butyl rubber will degrade under gamma ray irradiation through main chain scission while in contrast a chlorinated butyl rubber possesses considerable crosslinking potential. The addition of the polythiols of this invention do not change the response of unhalogenated butyl rubber to radiation exposure. The samples irradiated are described in Table XVl.

EFFECTOF lRRADlATlON UPON REGULAR BUTYL RUBBER IN THE PRESENCE AND ABSENCE OF POLYTHIOETHER POLYTHIOL Butyl Rubber 268" 100 100 100 Zinc Oxide 5 5 5 Stearic Acid 1 l 1 E232311l Polythiol' 2.25 4.5

"'Butyl rubber of 450.000 viscosity average molecular wci hi having about 1.5 mole unsuturution and a Mooney Viscosity at 260F. of u out 55.

After irradiation, the polymers were still completely soluble in cyclohexane. whereas the halogenated butyl rubbers cured under similar circumstances showed improved crosslink density.

EXAMPLE l1 EFFECT OF OTHER POLYTHIOLS ON THE Co lRRADlATlON lNDUCED CROSSLINKING OE CHLORlNA'l-ED BUTYL RUBBER (parts by weight) 19 Test samples molded for 1 minute at 175F. were soluble in cyclohexane (mylar film protected). These films were exposed to Co radiation at 0.01 megarads per minute followed by crosslinked "network analysis by .the sample E1221 additive would be one of the preferred structures from the standpoint effectiveness per unit weight.

EXAMPLE 12 volume swelling in cyclohexan'e with the following listed results. The radiation cure of polybutadiene is well known in Physical Properties after C0 lrradia- Control 98-1 98-2 E1221 tion at:. 1 2 3 4 .5 6 7 Vol. Increase 1110 627 500 621 583 469 381 Soluble Polymer* 8.8 3.3 1.9 3.1 2.5 0.7 0

Me x 10- 37 10.3 14.8 13.0 9.4 6.7

XL/cc x 10' 0.69 1.79 2.57 1.84 2.05 2.87 4.0

(Orig. Pol. Vol.)

% Vol. Increase 764 506 397 492 399 409 305 Soluble Polymer 5.4 3.3 1.5 1.7 1.7 2.2 0.3

Mc x 10- I 21 10.5 7.2 10 7.2 7.3 4.7

XL/cc x 10'' 1.28 2.49 3.64 2.65 3.64 3.52 5.67

% Vol. Increase 684 450 450 366 Soluble Polymer 7.1 4.5 4.0 3.1

Mc X 10' 17 8.7 8.7 6.1

XL/cc x 10-" 1.49 3.0 3.01 4.25

Including polythiols.

Although the thioeth'er polythiol provides the greatest .the art. Two typical polybutadienes were radiation degme of radiation cum enhancemel" (based "P cured using the method of this invention by preparing i crosslmks P equlvalem compositions which were molded into test samples of 9 1 the pollythlols i f i i't i also 0.025 thick films by press molding at 175F. for 5 min- 8 ecuve lcce emtmg colgen e owes meg!- utes between Mylar film. All samples were hydrocarrad dose of 5.1, the E1221 addltlve enhanced gcnerhon so] bl t t t t E ated crosslinks up to sixfold while the other additives u e pnor 0 rd ma men Xposures registered a threefold improvement. The difference were made under a 30 MR/mmelectron beam acceler' may be associated with coagent solubility in the hydrocarbon polymer medium. These data also illustrate that ator for various dose levels. The results are shown in Table XVlll.

TABLE XVIII "*a butyl lithium polymerized polybutadicne which has an approximate structure: 1.4 cis. 4571 n- 1 .2 (vinyl pendant groups) butadicne addition.

polyhutadienc which has an approximate structure: 92% 1,4 cis with some 1.4 trans and 27 1,2 addition of butadicne. I

Firestone Dione Phillips Cis 4 PB Physical Properties after irradiation at: 5 MR Volume Increase 1002 353 258 1115 594 522 Soluble 9.0 2.2 1.7 11.1 7.4 7.1 Me X 10 (gel) 24 5 3.1 29 11 9.1 XL/ce X 1019 1.06 5.57 9.25 0.96 2.41 2.97

' (Orig. Pol. Vol.) 10 MR Volume Increase 505 271 221 576 361 328 Soluble 3.3 1.1 1.3 5.5 4.3 4.5 Mr: X 10' 8.4 3.3 2.5 10.4 5.2 4.5 XL/cc X 10" 3.19 8.3 11.35 2.55 5.3 6.11 15 MR Volume Increase 395 256 209 445 298 274 Soluble 1.8 0.9 1.3 4.3 3.4 3.6 Mc X 10' 6.0 3.1 2.3 7.2 3.9 3.4 XL/Cc X 10"" 4.71 9.51 12.6 3.92 7.14 8.1

Columns 1 and 4 are the respective polymer controls with no additives, columns 2 and have just the 3 percent of added El,262 V11 and columns 3 and 6 are further supported with zinc oxide and stearic acid. The accelerating effect for either elastomer by the PTEPT is marked with further enhancement by zinc oxide, stearic acid on the basis of any network parameter. This is shown by a reduction in volume swelling and soluble polymer as well as by a smaller molecular weight between effective crosslinks. Crosslink density is increased manyfold, as much as nine times for the Diene 55 at 5 MR exposure. The Diene 55 is more responsive presumably due to its larger amount of vinyl pendant groups. Data of this nature reveal the tremendous potential for the precuring of polydiene elastomers at markedly reduced radiation dose levels.

The olefinically unsaturated polymers of this invention may be sulfur vulcanized subsequent to radiation curing. The radiation precure has process advantages where handling of shapes on sheets is necessary prior to vulcanization. Sulfur vulcanization may be carried out using free sulfur and accelerators or in the absence of free sulfur using sulfur donors. The term sulfur vulcanized means vulcanization using either free sulfur or sulfur donors. Both methods of sulfur vulcanization are well known to the art and are discussed in detail in Chapters 2 and 3 of Vulcanization and Vulcanizing Agenrs, W. Hofmann, Palmerton Publishing C0,, New York (1967) incorporated herein by reference.

lllustrative examples of accelerators which may be used in the sulfur cure of polymers of this invention are dithiocarbamates, xanthates, thiurams, thiazoles, aldehyde-amine accelerators and basic accelerators such as guanidine accelerators. Specific examples of these accelerators are zinc dimethyl dithiocarbamate, zinc diethyl dithiocarbamate, ammonium-N-pentamethylene dithiocarbamate, tellurium diethyl dithiocarbamate, sodium isopropyl xanthate, zinc butyl xanthate, tetramethyl thiuram monosulfide, tetraethyl thiuram disul fide, dimethyl diphenyl thiuram disulfide, 2 mercaptobenzothiazole, etc.

It will be evident to those skilled in the art that the aforegoing examples demonstrated that polythiols having at least three thiol groups which are effective cure enhancers for the radiation curing of SBR-polybutadiene and chlorinated butyl rubber. These polythiols preferably have a molecular weight of at least 150 and preferably less than 3,000, more preferably about 200 to 1.000, most preferably about250 to about 400. The preferred polythiols are hydrocarbon polythioether polythiols. These cure enhancers are incorporated in the polymer at about 0.5 to about 6 parts per hundred (phr) by weight of the rubber. more preferably about l to about 3 phr.

Useful dithiols are hydrocarbon dithiols such as dodecane dithiol, dipentene dimercaptan. vinyl cyclohexene dimercaptan. Such dithiols are generally prepared by reacting a large excess of hydrogen sulfide with a diolefm or acetylene.

More preferred dithiols are thioether dithiols such as those derived by the reaction of excess hydrogen sultide or dithiols with diunsaturates, such as ethylene diacrylate and methylacetylene.

Even more preferred are trithiols such as hydrocarbon trithiols exemplified by triallyl cyanurate trimercaptan, triacryloyl triazine trimercaptan, cyclododecatriene trimercaptan, trivinyl cyclohexane trimercaptan,

vinyl butyl acetylene trimercaptan. Such trithiols are prepared by reacting a very large excess of hydrogen sulfide with a triunsaturated compound.

Also highly preferred are polythiols containing more than three thiol groups such as a low molecular weight polybutadiene polymercaptan.

Most preferred are thioether trithiols, tetrathiols and thioethers having an even higher thiol functionality.

Such compounds are prepared by the addition of excess hydrogen sulfide andexcess dithiols to trienes and polyenes. Especially preferred species are derived by the addition of H 8 and alkylene dithiols to trienes, especially trivinyl compounds. Thioether polythiols in general contain one, two, three or more thioether groups. The number of thioether groups is about 1 to 100, preferably 1 to 25, more preferably 1 to 10, most preferably 1 to 5. The number of thioether groups is limited by economic considerations since these polythiols are used on an equivalent thiol basis.

Although it has been shown that conventional butyl rubber of low unsaturation cannot be radiation cured, the present invention is applicable to the more highly unsaturated isoolefin-conjugated diolefm polymers; see US. Pat. application Ser. No. 151,038, filed June 8, 1971, incorporated herein by reference for a description ofthese polymers and their method of preparation.

The improved radiation curing process of this invention is applicable to any olefinically unsaturated elastomer which has a number average molecular weight (M,,) of at least 10,000 and contains (1) at least 15 mole percent unsaturation or (2) at least 0.5 mole percent'halogen, e.g., chlorine in the allylic form. Both types of functionality may be present in the same polymer. For example, halogenated conjugated dienes, polymers andcopolymers, e.g. polychloroprene, contain both the requisite level of unsaturation and the allylic halogen.

The elastomers suitable for use in the practice of this invention preferably have a M,, of at least 50,000, more preferably at least 100,000, most preferably at least 120,000.

What is claimed is:

1. In a process for the curing of an elastomer having a number average molecular weight of at least 50,000 using ionizing radiation, wherein said elastomer is an ol'efinically unsaturated elastomer having 1) at least 15 mole unsaturation the improvement which comprises incorporating into the elastomer vulcanization mixture about 0.6 to about 6 parts per hundred by weight of elastomer of a cure enhancer comprising a hydrocarbon polythiol having about 3 to about 5 thiol groups and a molecular weight of about 200-1,000 selected from the group consisting of (1) the reaction product of cyclododecatriene and 1,3 propane dithiol, (2) the reaction product of trivinylcyclohexane and H 8, (3) the reaction product of trivinylcyclohexane and ethane dithiol, or (4) the reaction product of trivinylcyclohexane and 1,4 butanedithiol.

2. The process of claim 1 wherein the elastomer is styrene-butadiene rubber or polybutadiene.

3. The process of claim I wherein the cure enhancer has a molecular weight of about 250 to about 4,000.

4. The process of claim 1 wherein the cure enhancer is incorporated into the elastomer at about 0.5 to about 6 parts by weight per one hundred parts of elastomer.

5. The process of claim 4 wherein the cure enhancer is incorporated at about 2 to about 4- parts by weight per parts of elastomer.

6. The process of claim 1 wherein thej ela stomer" is styrene-butadiene rubber and the cure enhancer is l) the reaction product of trivinylcyclohexaneand H 8 or (2) the reaction product of trivinylcyclohexane and l ,3 propanedithiol. l

7. The composition prepared by the process of claim 1.

8. The sulfur vulcanized composition of the product of claim 7.

9. The process of claim 1 wherein the cure enhancer is the reaction product of cyclododecatriene and L3 propane dithiol.

10. The process of claim 1 wherein the curing enhancer is the reaction product of trivinylcyclohexane and H 8.

' II. The process ofclaim 1 wherein the cure enhancer is the reaction product of trivinylcyclohexane and ethane dithiol.

12. The process of claim I wherein the cure enhancer is the reaction product of trivinylcyclohexane and 1,4

butanedithiol'.

l4. Theprocess of claim I wherein the molecular weight of the thiol is about 150 to about 3,000.

-l5. The process of claim 1 wherein the curing enhancer is normally liquid at ambient temperature. 

2. The process of claim 1 wherein the elastomer is styrene-butadiene rubber or polybutadiene.
 3. The process of claim 1 wherein the cure enhancer has a molecular weight of about 250 to about 4,000.
 4. The process of claim 1 wherein the cure enhancer is incorporated into the elastomer at about 0.5 to about 6 parts by weight per one hundred parts of elastomer.
 5. The process of claim 4 wherein the cure enhancer is incorporated at about 2 to about 4 parts by weight per 100 parts of elastomer.
 6. The process of claim 1 wherein the elastomer is styrene-butadiene rubber and the cure enhancer is (1) the reaction product of trivinylcyclohexane and H2S or (2) the reaction product of trivinylcyclohexane and 1,3 propanedithiol.
 7. The composition prepared by the process of claim
 1. 8. The sulfur vulcanized composition of the product of claim
 7. 9. The process of claim 1 wherein the cure enhancer is the reaction product of cyclododecatriene and 1,3 propane dithiol.
 10. The process of claim 1 wherein the curing enhancer is the reaction product of trivinylcyclohexane and H2S.
 11. The process of claim 1 wherein the cure enhancer is the reaction product of trivinylcyclohexane and ethane dithiol.
 12. The process of claim 1 wherein the cure enhancer is the reaction product of trivinylcyclohexane and 1,4 butanedithiol.
 13. The process of claim 1 wherein the molecular weight of the elastomer is at least 100,000.
 14. The process of claim 1 wherein the molecular weight of the thiol is about 150 to about 3,000.
 15. The process of claim 1 wherein the curing enhancer is normally liquid at ambient temperature. 